Buss, who’s a psychology professor, went to a party at another professor’s house one day and found his friend enraged because his wife had criticised his shirt in front of other people.

“I’m going to kill her,” the ordinarily mild-mannered professor friend said through his teeth.

Later that night, Buss’ professor friend called him and said he had to get out of house.

“If I don’t leave my house right now, I’m going to kill her,” the friend said, according to Buss.

This event got Buss thinking about whether regular people fantasize about murder.

A few years later, he surveyed members of an introduction to psychology class about whether they had ever fantasized about murder. He was shocked when the vast majority of his class – roughly 75 per cent – said they did have homicidal fantasies.

Buss then surveyed 5,000 people around the world, finding 91 per cent of men and 84 per cent of women thought about ending somebody else’s life. He also analysed FBI files of more than 400,000 murders.

So, why does the thought of murder enter people’s minds so easily?

“Killing is fundamentally in our nature because over the eons of human evolution murder was so surprisingly beneficial in the intense game of reproductive competition,” Buss told the University of Texas at Austin, where he’s a professor.

Men in particular are more likely to kill if they’re humiliated, which could hurt their ability to attract a mate, according to Buss.

For instance, ex-LAPD officer Christopher Dorner was fired from his job as a police officer and honorably discharged from the U.S. Navy just days before his alleged rampage. It’s unclear what kind of impression he made on those who knew him.

But in most pictures, he has a huge smile on his face, and a former LAPD captain called Dorner’s firing “very, very ugly.”

Dorner died during his standoff with authorities, so we’ll never know why he killed. Some psychologists believe he was a narcissist. Regardless, it’s clear that he seemed totally normal before he murdered people.

“Though we might like to think that murderers are either pathological misfits or hardened criminals who,” Buss told UT, “the vast majority of murders are committed by people who until the day they kill, seem perfectly normal.”